We propose to continue research into the natural Circular Dichroism (CD) and Magnetic Circular Dichroism (MCD) of metalloproteins in the iron-sulfur, heme and copper categories. The CD and MCD chiroptical techniques allow the electronic structure, and thereby also the chemical composition and geometrical structure of metal-containing active site moieties of metalloproteins to be directly probed. The research makes use of comprehensive, state-of-the-art instrumentation for the measurement of CD and MCD across a wide spectral region from the medium lR to the vacuum UV. In addition, a new high field (7 Tesla) split-coil superconducting magnet system permitting variable sample temperatures over the range 1.6 degrees K - 300 degrees K will enormously increase the variety and in formation content of MCD experiments. This proposal emphasizes variable temperature MCD studies in the liquid helium temperature range. In the case of iron-sulfur proteins, we propose, first, to study a wide variety of simple 1-Fe, 2-Fe, 4-Fe and 8-Fe ferredoxins at low temperatures. This will both further establish the diagnostic utility of CD and MCD in identifying iron-sulfur cluster type and oxidation level and provide optimal data for evaluation of theoretical models of the electronic structures of these clusters. Subsequently, "complex" iron-sulfur proteins will be studied, including azotobacter vinelandii Ferredoxin I and Mycobacterium flavum ferredoxin. Initial emphasis in the area of heme proteins will focus on thorough low-temperature studies of well-characterized heme ligation, oxidation and spin-states in much studied proteins such as myoglobin, cytochrome c etc. As comprehensive a description as possible of the electronic states and transitions of these systems will be attempted. Less well understood heme states will then be studied, including the intermediate spin state of cytochrome c' and the Fe(IV) oxidation level.